The index of refraction of the air surrounding the interferometer must be considered when making precise interferometric length or angle measurements since the index of refraction is included directly in the result of the measurement. German patent application DE-OS No. 34 01 900 discloses an interferometer with a reference system with which the index of refraction as well as a length or an angle can be measured simultaneously. For this purpose, an evacuable chamber is arranged in one component beam of the comparison arm and the reflection element of this component beam or of the other component beam is moved for length or angle measurements.
A disadvantage with this known interferometer is that the precision of the length measurement and of the index of refraction measurement is inadequate for very high requirements.
A publication entitled "Ein Meter-Komparator fur interferometrische Langenbestimmungen in Vakuum-Wellenlange" of Walter Kinder in the "Zeiss Werkzeitschrift" (Volume 43, 1962) pages 3 to 11, discloses a meter comparator for interferometric length determinations in vacuum wavelengths wherein the sensitivity of the determination of the index of refraction is doubled in that two chamber pairs are so utilized that two interferometric systems operate opposite to each other. The length measurement is achieved in a conventional manner with beams independent of the determination of the index of refraction.